1. Field of the Invention
The present invention relates to an SAW (Surface Acoustic Wave) filter, and more particularly, it relates to an SAW filter which has an improved electrode structure in relation to interdigital transducers (hereinafter referred to as IDTs), thereby having an improved thermal shock resistance.
2. Description of the Background Art
FIG. 1 is a typical plan view showing a conventional SAW filter 1. The SAW filter 1 comprises an SAW substrate 2 which is made of a piezoelectric single crystal such as LiNbO.sub.3 or lead zirconate titanate (PZT) piezoelectric ceramics, and input and output IDTs 3 and 4 which are located thereon at a prescribed distance from each other. The IDTs 3 and 4 have pairs of comb electrodes 3a, 3b, 4a and 4b provided with plural electrode fingers which are interdigitated with each other, respectively. These comb electrodes 3a, 3b, 4a and 4b are formed by connecting the plural electrode fingers to bus bars 5 to 8, respectively. The bus bars 5 to 8 are provided on end portions thereof with terminal electrodes 9a, 9b, 10a and 10b for electric connection with the exterior, respectively. Numerals 11 and 12 denote wave absorbing materials which are made of silicone rubber or the like.
In this SAW filter 1, surface waves are excited by an input signal in the input IDT 3, so that the excited surface waves are propagated toward the output IDT 4. An output signal which is based on the propagated surface waves is extracted from the output IDT 4.
The IDTs 3 and 4 are provided with the electrode fingers on end portions along the surface wave propagation direction, while no electrode fingers are provided on the adjacent portion of the SAW substrate 2. Due to such discontinuity between the IDTs 3, 4 and the portion adjacent thereto, an electric field which is generated upon development of a potential difference across the electrode fingers is rendered so discontinuous that the electric field is particularly strong between the electrode fingers provided on the end portions of the IDTs 3 and 4.
When the SAW substrate 2 is made of a pyroelectric material such as LiNbO.sub.3 or PZT and charges are generated due to application of a thermal shock, the electrode fingers provided on the end portions of the IDTs 3 and 4 are easily damaged in discharge. Such damage of the electrode fingers on the end portions of the IDTs 3 and 4 is now described with reference to FIG. 2.
FIG. 2 is an enlarged view illustrating such a state that the electrode fingers of the comb electrodes 3a and 3b are damaged in one end portion of the IDT 3. The materials forming the electrode fingers of the comb electrodes 3a and 3b provided on the end portion of the IDT 3 are melted by discharge as shown by arrows A and B, and broken. Even if the electrode fingers are not yet broken by such melting of the materials, the electrode fingers may be roughened similarly to a case of migration.